The present invention is a process for converting a high-boiling component resulting from the reaction of an organochloride with silicon into commercially more desirable monosilanes. The process comprises contacting the high-boiling component with hydrogen gas at a temperature within a range having a lower limit greater than 250.degree. C. and an upper limit of 1000.degree. C. Yield of the present process may be improved by use of a catalyst selected from a group consisting of activated carbon, platinum metal, platinum supported on alumina, palladium supported on carbon, SbCl.sub.5, H.sub.2 PtCl.sub.6, BCl.sub.3, AlCl.sub.3, and AlCl.sub.3 supported on a support material selected from a group consisting of carbon. alumina, and silica. In a preferred embodiment of the present process the process is run at a pressure within a range of about 250-1000 psig. A significant advantage of the present process is that it is preferential for the production of diorganodichlorosilane in relation to organotrichlorosilane in the monosilane product.
The high-boiling component useful in the present process results from a process typically referred to as the "Direct Process", where an organohalide is reacted with silicon metalloid in the presence of a suitable catalyst to form monosilanes. The Direct Process as described by, for example, Rochow, U.S. Pat. No. 2,380,995, issued Aug. 7, 1945, and Barry et al., U.S. Pat. No. 2,488,487, issued Nov. 15, 1949, is the main commercial process by which organohalosilanes (i.e. monosilanes), for example, dimethyldichlorosilane and trimethylchlorosilane are formed. These organohalosilanes are reactive compounds which can undergo numerous reactions to form a variety of useful silicon containing compounds and polymers. In typical commercial direct processes the process is optimized to produce the diorganodihalosilane, since this monosilane can be hydrolyzed to form polysiloxane polymers having a broad range of commercial applications. Polysiloxane polymers are useful, for example, as heat transfer fluids, lubricants, and the like and can be further processed, for example, to form silicone elastomers, resins, sealants, and adhesives.
Operation of the Direct Process results not only in the production of the desirable monosilanes, but also in a high boiling component typically considered to be all materials with a boiling point higher than the particular diorganodihalosilane produced in the process. The high-boiling component is a complex mixture that includes compounds containing SiSi, SiOSi, SiCSi, SiCCSi, and SiCCCSi linkages in the molecules. Some typical compounds found in a high-boiling component are described, for example, in Mohler et al., U.S. Pat. No. 2,598,435, issued May 27, 1952, and Barry et al., U.S. Pat. No. 2,681,355, issued Jun. 15, 1954. The high-boiling component may also comprise silicon containing solids and soluble and insoluble compounds of copper, aluminum, and zinc.
In current commercial operations for performing the Direct Process, the high-boiling component can constitute as much as ten percent of the resultant product. Therefore, it is desirable to convert the high-boiling component into commercially desirable products to both reduce low value by-products and to improve raw material utilization.
Mohler, U.S. Pat. No. 2,598,435, issued May 27, 1952, describes a process for converting methylpolysilanes present in a Direct Process residue to monosilanes, the process comprises heating the residue at a temperature above 250.degree. C. and below the decomposition point of the formed monosilanes.
Barry. U.S. Pat. No. 2,681,355, issued Jun. 15, 1954, observed that the process taught in Mohler, U.S. Pat. No. 2,598,435, can result in significant coking of the reactor making the process unsuitable for commercial cracking processes. Barry, supra, teaches that this coking can be reduced if the Direct Process residue is contacted with at least four percent by weight hydrogen chloride at a temperature from 200.degree. C. to 900.degree. C. Barry also suggests that the process can be run in a reactor packed with either an inert material such as quartz or a catalytic material such as activated alumina or silica alumina.
Bluestein, U.S. Pat. No. 2,709,176, issued May 25, 1955, reports a process for converting the polysilanes present in a Direct Process residue into monosilanes by the use of a tertiary organic amine catalyst. Bluestein reports that when the Direct Process residue is contacted with a hydrogen halide and a tertiary organic amine catalyst, the process can be conducted at temperatures of about 75.degree. C. to 150.degree. C. with acceptable yields of monosilanes being obtained.
Gilbert. U.S. Pat. No. 2,842,580, issued Jul. 8, 1958, reports a process for converting the polysilanes present in a Direct Process residue into monosilanes by the use of quaternary ammonium halide and quaternary phosphonium halide compounds as catalysts. The process of Gilbert is run in the absence of hydrogen chloride, as described by Bluestein supra, and is reported to provide monosilanes with reduced levels of hydrogen bonded to the silicon atoms.
Atwell et al., U.S. Pat. No. 3,639,105, issued Feb. 1, 1972, describe a process where hydrosilanes are produced by contacting disilane with hydrogen gas under pressure. The resulting mixture is heated in the presence of a transition metal catalyst at temperatures within a range of 25.degree. C. to 250.degree. C. Atwell et al. teach that at temperatures in excess of 250.degree. C. catalyst and/or disilane decomposition tends to occur which deleteriously affects the reaction.
Calas et al., U.S. Pat. No. 4,059,608, teach a process for hydrogenating disilanes, where a catalyst system containing an aprotic compound and a nickel catalyst is used. Calas et al. teach the process can be conducted at a temperature within a range of 50 to 200.degree. C. In the provided examples the ratio of dimethyldichlorosilane to methytrichlorosilane varies between 0.7 to 1.0.
Neale, U.S. Pat. No. 4,079,071, issued Mar. 14, 1978, teaches a process for preparing high yields of hydrosilanes by reacting methylchloropolysilanes with hydrogen gas under pressure at a temperature of from about 25.degree. C. to about 350.degree. C. in the presence of a copper catalyst.
Takeda et al.. Kogyo Kagaku zasshi (Journal of Industrial Chemistry), Vol. 60, No. 11, p. 1392-1395 (1957), described the use of alumina, carbon, and pumice as catalyst for the catalytic cracking of disilane in a hydrogen stream.
An objective of the present process is to provide a process for the conversion of a high-boiling component from the direct process to monosilanes. A second objective is to provide a process preferential for the production of diorganodichlorosilane in relation to organotrichlorosilane in the monosilane product.